1. Field of the Invention
The present invention relates in general to an electrochemical device, and more particularly to an electrochemical device with an electrochemical element having an electrochemical cell which includes a solid electrolyte body and a heater formed integrally with the cell.
2. Discussion of the Prior Art
An electrochemical device which uses a zirconia oxygen-ion conductive solid electrolyte body and operates according to the principle of an oxygen concentration cell is known as an oxygen sensor for measuring or determining the concentration of oxygen in exhaust gases emitted by a vehicle internal combustion engine, or in combustion gases in industrial furnaces and boilers. In the art of controlling the combustion of an air-fuel mixture in an internal combustion engine for a motor vehicle, for example, the oxygen concentration in the exhaust gases is generally detected by such an electrochemical oxygen sensing device, in order to determine the air/fuel ratio of the air-fuel mixture which has a known relationship with the oxygen concentration. Based on the determined air/fuel ratio of the air-fuel mixture, an amount of supply of the fuel to the engine is controlled in a feedback manner, for accurately maintaining the air/fuel ratio of the mixture at an optimum level.
Usually, it is required to provide such an electrochemical device with a suitable heater to maintain the electrodes and solid electrolyte body of an electrochemical cell of the sensing element at an optimum elevated operating temperature, so that the electrochemical device may be operated so as to exactly determine the oxygen concentration of a gas to be measured, even when the temperature of the measurement gas is comparatively low. An example of a heater-built-in electrochemical element is disclosed in U.S. Pat. No. 4,300,990, wherein a heater layer incorporating a heat-generating element is formed on one of opposite major surfaces of an electrochemical cell consisting of a solid electrolyte body and a plurality of electrodes, such that the heat-generating element produces heat for heating at least a portion of the cell at which the electrodes are disposed.
Described more specifically, a heater-built-in electrochemical element as indicated above includes a planar body of an oxygen-ion conductive solid electrolyte, a plurality of electrodes formed on the planar solid electrolyte body, an electrically insulating ceramic layer formed on the planar solid electrolyte body, and a heater layer which is electrically insulated from the solid electrolyte body by the ceramic layer. The heat-generating element of the heater layer is connected to a dc voltage source of 10-20 V so that a heater current is applied to energize the heat-generating element, whereby the electrochemical element is heated by heat produced by the energized heat-generating element. For example, the electrochemical element is heated to a temperature not lower than about 300.degree. C. At this high temperature, the current applied to the heat-generating element may leak from a high-potential terminal of the heat-generating element into the solid electrolyte body, through the insulating ceramic layer, and back to a low-potential terminal of the heat-generating element, again through the insulating ceramic layer. This leak current causes reduction and consequently deterioration of a portion of the insulating ceramic layer adjacent to the low-potential terminal of the heat-generating element, and a portion of the solid electrolyte body which contacts the above-indicated portion of the ceramic layer. Thus, the known heater-built-in electrochemical element suffers from shortened life expectancy due to such deterioration of the ceramic layer and solid electrolyte body.
In view of the above problem of the heater current leakage and resultant reduction in the service life of the heater, it has been proposed to electrically connect one of the electrodes on the solid electrolyte body, to the low-potential terminal of the dc power supply for the heater as disclosed in U.S. Pat. Nos. 4,798,693 and 4,728,411. While this arrangement prevents the conventionally experienced problem, the proposed wiring arrangement requires a bi-polar power source for the electrochemical cell (solid electrolyte body and electrodes) to be able to effect a bi-directional oxygen pumping operation. Accordingly, the cost of the electrical circuitry of the electrochemical device undesirably increases, where the electrochemical device is used in automotive vehicles or in other environments in which only a unipolar power source is available.